Since the 1960s it is known in the art of steel making to continuously cast near-net-shape sections for rolling e.g. I-beams or H-beams. These near-net-shape sections are called beam blanks. They have a substantially H-shaped cross-section with a web centrally arranged between two lateral flanges. Today such beam blanks are even used to roll Z-shaped sheet-piles and other steel sections.
Beam blanks are produced by continuous casting, i.e. liquid steel is continuously fed into a short, water-cooled copper mould with an open vertical casting channel, and a beam blank strand, which has the final cross section of the beam blank to be produced, is continuously withdrawn from this mould. At the outlet of the continuous casting mould the continuous beam blank strand has only a thin solidified outer shell enveloping a liquid steel core. Solidification of the beam blank strand is then continued by spray cooling, wherein a cooling fluid, generally water or an air-water-mist, is sprayed onto the perimeter surfaces of the beam blank strand. This spray cooling takes place in a secondary cooling zone beneath the continuous casting mould. In this secondary cooling zone the beam blank strand is guided in a vertical casting plane along a curved path, with its web being perpendicular to the vertical casting plane. An extraction and straightening device, which is located downstream of the secondary cooling zone, straightens the bent beam blank strand, prior to pushing it onto a horizontal run-out table, where beam blanks of a desired length are cut from the continuous beam blank strand.
It is well known in the art of continuous casting that a good control of the secondary cooling of the strand is of utmost importance for the final quality of the cast product. It is indeed this secondary cooling that allows to control temperature evolution in the strand during its final solidification, thereby allowing to control the microstructure of the cast product.
While spray cooling in the secondary cooling zone of a continuous casting line allows a rather good control of temperature evolution during solidification of billets, blooms or slabs, this is not yet the case for beam blanks. Indeed, due to the fact that beam blanks have—in comparison with billets, blooms and slabs—a relatively complex cross-section (comprising elements of different thickness, orientation and perimeter surface to volume ratio), it is very difficult to closely control the evolution of the temperature profile in a beam blank by spray cooling. A more or less uniform spray cooling of all the perimeter surfaces of the beam blank will for example inevitably result in an overcooling of the flanges. However, trying to avoid an overcooling of the flanges by reducing direct spray cooling of the flange surfaces, results in an insufficient cooling of the massive joining portions between the flanges and the web, which still enclose important liquid steel pockets. The consequence of an insufficient cooling of this liquid steel pockets is a bulging of the shell in the flange/web joining portions due to the internal pressure in the liquid steel pockets and an increased risk of a liquid steel break-through. In conclusion, optimising the secondary cooling of a beam blank is a rather complex problem, which has already been and still is the object of numerous research programs. However, despite the use of sophisticated computer programs for selectively controlling spray cooling of the different zones of the beam blank in function of various casting parameters, present beam blanks still tend to have major shortcomings.
One of these shortcomings of present beam blanks is the presence of transverse cracks in the intrados flange tips. These transverse cracks appear in the intrados flange tips when the beam blank is straightened in the straightening device. They are observed in particular, but not exclusively, in large section and high strength beam blanks. Although it is very likely that these transverse defects are due to an undesired quench of the flange tips during secondary cooling, it has not yet been possible to reliably avoid these cracks, e.g. by a better control of the secondary spray cooling. In this context it has to be pointed out that it is particularly problematic to control secondary cooling of the intrados flange tips, because these flange tips are not only cooled by the cooling fluid that is directly sprayed onto the intrados portions of the flanges, but also by the cooling fluid that is sprayed onto the intrados side of the web and of the web/flange joining portions. Indeed, at least part of this intrados cooling fluid flows laterally over the intrados flange tips, thus causing an undesired vigorous cooling of the latter. In order to reduce risk of quenching the flange tips, spray cooling of the intrados side of the beam blank strand should therefore be generally limited, but this would result in other problems, as e.g. a bulging of the shell on the intrados side of the flange/web joining portions.
JP-A-10263752 is concerned with the prevention of warping at the flange part during the continuous casting of beam blanks. To prevent this flange warping this document suggests to eliminate the temperature difference between the flange surfaces and the last solidified part of the beam blank by heating the flange surfaces of the beam blank up to 900-950° C. until the beam blank reaches final solidification or shortly thereafter. The Japanese document contains however no teaching about avoiding transverse cracks in the intrados flange tips during the straightening of the beam blank.